Conventionally, in a keyboard that is used in a portable personal computer or the like, the so-called operability, in which a key top descends without tilting regardless of which portion of the key top is pushed, is ensured. To this end, a conventional key switch structure has a link mechanism at the lower portion of the key top. For example, there is the structure disclosed in Japanese Patent Application Laid-Open No. 2001-229764 as a key switch structure having a link mechanism at the lower portion of the key top.
The conventional key switch structure, that is equipped with a link mechanism and is disclosed in the aforementioned document, is shown in FIG. 16. In FIG. 16, a conventional key switch 10 is structured from a key top 11, a first link member 12 that is provided so as to be able to rotate with respect to the key top 11, a second link member 13 that is provided so as to be able to swing with respect to the key top 11, a rubber dome (elastic restoring member) 14 that bends due to the key top 11 being pushed-down, and restores the key top 11 to the original position when the push-down force is eliminated, a holder 15 that holds the first, second link members 12, 13, a membrane sheet 16 having a contact portion 16a directly beneath the rubber dome 14, and a back plate 17 to which the holder 15 is fixed. A link mechanism is structured by the first link member 12 and the second link member 13.
Further, an embossed portion 17a that projects upward is formed at the back plate 17, and this embossed portion 17a is set in a through-hole 16b that is formed in the membrane sheet 16. Further, a pin 15a for welding is formed at the lower portion of the holder 15, and this pin 15a for welding is set in a hole 17b that is formed in the embossed portion 17a of the back plate 17. In the state in which the pin 15a for welding is set in the hole 17b, the back plate 17 is welded to the holder 15 with the membrane sheet 16 sandwiched therebetween.
In the above-described key switch structure, even if an end portion of the key top 11 is pushed-down, the key top 11 descends while the horizontal state is maintained by the link mechanism that is formed from the first, second link members 12, 13. For example, when the position of the end portion of the key top 11 shown by arrow A in FIG. 16 is pushed-down, first, the right end portion of the first link member 12 descends. The left end portion of the first link member 12 moves toward the left side. Due thereto, the central portion of the first link member 12 descends, and the second link member 13, that is connected to the first link member 12 at the central portion, also descends. Due to the descending of the second link member 13, the left end portion of the key top 11 also descends. Due to the key top 11 descending while maintaining a horizontal state even if an end portion of the key top 11 is pushed-down in this way, it is made such that there is no difference in the operation sensation depending on the push-down position, i.e., such that the operability is ensured.